This project involves the genetic dissection of mammalian cell division rates. Cells were taken from the liver and lung of a single male hamster newborn. Three cultures of lung and one of liver each expressing G1 (G1 plus cells) were fused with other established G1 plus cell lines. In a number of crosses, the hybrids lacked G1 (G1 minus cells) suggesting that the cells were expressing G1 for different reasons. Previous work had shown that the G1 minus yields G1 plus phenotypic change could be induced by partially inhibiting protein synthesis. At present, we are asking if a G1 plus yields G1 minus change can be induced by slowing down the division cycle (S phase). Preliminary results suggest that by partially inhibiting S traverse with low levels of hydroxyurea the "growth cycle" can keep pace with the now slower division cycle and result in a cell that has the same generation time but no G1 phase. Such results are consistent with the notion that the G1 interval as a period of time is not based on numerous events that must occur in G1 (between M and S) but is rather a reflection of insufficient growth.